This chapter describes the structure of the gut and explains how its various parts perform different phases of the digestive process. To top things off, the liver – the largest and arguably most important organ in the body – is discussed and guidelines are given on how you can protect it.
Some people find the word gut disagreeable and perhaps a little coarse. In fact, it is a proper descriptive medical term and the title of a highly respected medical journal. In medical usage, gut refers to the small and large intestines, gastrointestinal describes the gut plus the stomach, and digestive system covers all parts of the body involved in the breakdown of food and absorption of nutrients, including the mouth, teeth, liver and pancreas. In this book, the three terms are used interchangeably.
It is not just words that make people feel uneasy; many of us find the need to describe the processes of the gut hugely embarrassing. Perhaps it is the influence of television chat shows and medical dramas, but talking about personal sexual or mental problems often comes easier than discussing the more awkward subject of digestion. Many older people find it rude to speak of ‘fart’ or ‘faeces’. Fortunately, younger people seem less affected by these inhibitions. This text provides a balance of terms that will suit all. A later section suggests ways you can ask for help and accurately describe your symptoms with minimal embarrassment (see here).
how the gut controls your health
The gut is the gatekeeper between nutrients in the food we eat and the internal working of the body. When it is damaged by illness or injury, the inward flow of vital nutrients dwindles. The heart, muscles, blood and bones are denied the carbohydrates, proteins, vitamins and minerals needed for energy, repair and immunity. The healthiest diet, brimming with the finest foods, is useless to the body if all parts of the digestive system are not functioning in harmony.
STRUCTURE OF THE GUT
The gut is a hollow tube running through the body’s core – top to bottom – from the mouth to the anus. Many writers compare it with a hole in a doughnut. This imagery suggests that the gut is an inert space, passively allowing the flow of food and drink from one end to the other. Nothing could be more misleading. The gut is a miraculous structure containing highly specialized parts capable of grinding, liquefying, extraction, absorption and elimination. Its cellular lining controls its many chemical processes, while its outer muscular wall kneads, moves and stores the material passing through it.
Modifications in the structure of the gut’s lining are astounding. For example, the mouth has a smooth lining, but contains the tongue and teeth needed to chew and crush the food. In sharp contrast, the lining of the small intestine is deeply folded and covered with millions of tiny, finger-like protrusions call villi. The villi are packed with blood capillaries so close to their surface that proteins, sugars, fats and essential nutrients flow smoothly from the lumen of the gut into the bloodstream. Lower down the passageway, the digestive matter that remains after nutrients have been extracted comes in contact with friendly bacteria that bulk the mass. The walls of the gut remove excess water, and the remaining unwanted material (faeces) is stored before being expelled from the body. A problem anywhere along this chain of events can result in inefficiency or failure of the entire system. It is easy to see why the health of this low tube, with all its parts, determines how well your body functions and how well you feel.
The healthy gut is highly mobile, although we are usually unaware of its activity. Muscular contractions (peristalsis) mix and move food along the tube as digestion progresses. We are more aware of the gut’s motility when things go wrong. For example, if contractions are slowed – by drugs, for example – constipation may be a result. Or, if their speed is increased due to stress, excessive amounts of caffeine, food poisoning or medication – a bout of diarrhoea may be in the offing.
The major divisions of the digestive system are the mouth, oesophagus, stomach, duodenum, small intestine, large intestine (colon), rectum and anus. Along its path, several glands provide chemicals (enzymes) that aid the breakdown of certain food components: these are the salivary glands in the mouth, the pancreas and the gall bladder. Parts of the gut contain cells that secrete other chemicals needed for digestion. The stomach, for example, contains cells that secrete a strong acid.
In a healthy person, the contents from a meal take between 15 and 18 hours to pass from one end of the digestive tube to the other. Three to five hours are spent in the stomach, and about four hours in the small intestine where absorption of nutrients takes place.
Digestion begins in the mouth, where food is crushed and ground by the teeth and mixed with saliva containing enzymes that start breaking down starchy foods into simple sugars. Proper chewing is important. Food needs to be properly shredded to give maximum exposure to digestive enzymes, and saliva needs a chance to be mixed into the mass of food. After being swallowed, food is carried by muscular waves down the oesophagus and emptied into the stomach. A muscular valve (rather like a rubber band) prevents food and stomach fluid returning up the oesophagus.
The stomach is a large muscular pouch, or bulge, in the gut where food is thoroughly mixed and kneaded. Little is absorbed in the stomach other than certain drugs and alcohol. The lining in this part of the digestive tube contains two types of specialized cells: one secretes powerful hydrochloric acid needed to liquefy the food, and the other produces thick mucus needed to protect the lining of the stomach from its own acid. If the mucous barrier breaks down, ulcers can develop. Despite its acidic environment, the stomach can be infected with a bacterium, Helicobacter pylori, which is present in the majority of ulcer patients. (Ulcer treatment now includes antibiotics that eliminate this unwelcome intruder and speed healing.) From the stomach, the products of digestion pass through another rubber-band-like constriction (the pyloric valve) at the bottom of the stomach and enter the most important part of the digestive system: the small intestine.
The small intestine is about 7.5 metres (25 feet) in length and has three distinguishable parts: the duodenum (separated from the stomach by the pyloric valve), the jejunum and the ileum. Each is modified to perform a specific biochemical process. The first step must be to neutralize the digestive material coming from the stomach to make it slightly alkaline. This occurs in the duodenum, where bile manufactured by the liver and stored in the gall bladder joins alkaline fluids from the pancreas (needed to break down fats) and flows into the gut through a delicate structure called the common bile duct. (The pancreas also produces insulin needed to control blood sugar levels, but that hormone is transferred directly from the pancreas into the bloodstream.) By this time, the environment of the gut is watery. Pancreatic enzymes continue working as the digestive matter enters the jejunum, where enzymes produced by cells in the gut wall complete the breakdown of carbohydrates into sugars, fats into fatty acids and glycerol, and proteins into amino acids. Along the way vitamins, minerals and other compounds are released from the digested remains of food. The breakdown of food into its useful parts is called digestion.
Millions of finger-like villi rise out of the intestinal wall and are washed by the nutrient slurry passing through. Each is covered by a thin membrane of cells encasing a twist of blood capillaries so small that red blood cells flow through single file. Individual molecules of protein (amino acids), sugar, vitamins and minerals pass easily from the slurry into the bloodstream, which carries them to the liver for further processing within the body (metabolism). This passage is the normal case, and is called absorption.